An engine may be periodically automatically stopped without an operator specifically requesting an engine stop. Stopping the engine during low torque demand conditions may conserve fuel. The engine may also be subsequently automatically restarted when an engine torque demand increases to propel a vehicle in which the engine operates. However, if a transmission coupled to the engine is in gear and engine speed is greater than a threshold speed during starting, more than a desired amount of engine torque may be transferred from the engine to vehicle wheels. Therefore, it may be desirable to control engine speed during starting. One way to control engine speed during starting is to retard spark timing. Yet, retarding spark timing may produce a misfire or partial combustion of cylinder gases. Similarly, the engine air fuel ratio may be adjusted leaner to reduce combustion torque and limit engine speed, but the engine may misfire and engine emissions may increase. Further, it may be particularly difficult to control engine speed for a port injected engine during run-up because injected fuel may or may not enter the cylinder.
The inventors herein have recognized the above-mentioned issues and have developed an engine starting method, comprising: starting an engine from stop via rotating the engine; and rotating a torque converter impeller speed from zero speed in response to a first predetermined number of engine combustion events since engine stop via engine torque and at least partially closing a torque converter disconnect clutch from an open state.
By adjusting a disconnect clutch during an engine start according to a number of combustion events since engine stop, it may be possible to limit the amount of engine torque transmitted to vehicle wheels while a transmission is in gear. For example, a disconnect clutch positioned between an engine and a transmission input shaft may be at least partially opened and closed during engine run-up (e.g., the time during engine starting where engine speed exceeds cranking speed and is less than engine idle speed) to limit torque transmitted by the engine to vehicle wheels. In one example, the disconnect clutch may be opened so that torque converter impeller speed does not exceed a threshold speed. In this way, torque transmitted through a transmission may be limited without adjusting combustion in engine cylinders. Consequently, the possibility of engine misfire may be reduced.
The present description may provide several advantages. In particular, the approach may improve engine starting by improving engine speed control during engine starting. Further, the approach may reduce engine emissions. Further still, the approach may improve vehicle drivability.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.